1. Field of the Invention
The present invention relates to novel, liquid, non-flammable dielectric compositions, and, more especially, to such dielectric compositions well suited as insulators/coolants for electrical transformers.
2. Description of the Prior Art
It is art recognized that the dielectric liquids utilized in transformers perform a dual function. On the one hand, they serve as insulating liquids and, in this respect, they must conform to certain pre-determined requirements regarding their electrical properties, especially their dielectric strength and loss factor. On the other hand, they concurrently serve as a cooling agent for the apparatus, and they must thus ensure excellent elimination and dissipation of the heat generated during operation of the transformer. This latter function cannot be successfully fulfilled unless the agent employed exhibits, under the very variable conditions of use of the transformer, a sufficiently low viscosity for the liquid as to enable same to readily dissipate the heat evolved. It is also art recognized that transformers may have to function at extremely low temperatures, for example, below 0.degree. C., and even as low as -40.degree. C. It is thus important that at these extreme temperatures the dielectric should remain a liquid having a sufficient fluidity and should, furthermore, not give rise to complete crystallization, or even to the simple deposition of crystals prone or apt to block or clog the pipelines and pumps which circulate the cooling liquid in certain types of apparatus. Moreover, the presence of crystals too is responsible for a considerable drop in the breakdown voltage, as a result of the electric field heterogeneity which they cause.
In addition to these properties, it is also necessary, for certain types of transformers, that the dielectric liquids should be non-flammable. In fact, under the conditions of operation of the transformers, a destruction of the dielectric can occur, with the production of an electric arc which may be of very high power. This breakdown arc decomposes the liquid or solid dielectrics and can ignite the liquid and/or the gases evolved, whether these are decomposition products of the dielectric or of the vapors thereof. It is thus important that the dielectric liquid and its vapors, or the decomposition gases produced in the event of a fault in operation of the apparatus, should not ignite. In general, this resistance to ignition is assessed in terms of the flash point or the ignition point of the liquid in question.
Numerous liquid dielectrics for transformers which, to a greater or lesser extent, exhibit all of the properties enumerated above have been proposed. Among these products, there may be mentioned the "askarels" which have proved most satisfactory and which are used most widely. Same are biphenyl or terphenyl chlorination products containing from 3 to 7 chlorine atoms, which are most frequently employed in the form of admixture with one other or with other chlorinated aromatic hydrocarbons, and especially with the trichlorobenzenes and tetrachlorobenzenes. In spite of their demonstrated value, these particular dielectrics exhibit the severe disadvantage that they cannot be degraded biochemically and are difficult to degrade chemically. This stability of the polychlorobiphenyls presents serious hazards from an environment pollution standpoint, such that a need is becoming increasingly more evident for products having as short a life as possible, in nature, as a result of being increasingly chemically or biochemically degradable. Such products which exhibit both the aforementioned technical properties, as well as enhanced degradability, were hitherto unknown to the industry.